Many lipids control diverse cellular processes related to cell proliferation, apoptosis, metabolism and migration. Lipids, such as phosphoinositides, sphingolipids, and fatty acids, their binding partners, and/or their downstream targets may constitute complex signaling networks that control these processes, whereby imbalances in these networks may contribute to the pathogenesis of human diseases, such as inflammation, cancer, diabetes, and metabolic diseases.
Since lipids are continuously produced, degraded, and transported in a tightly controlled manner, determining their spatio-temporal fluctuation is necessary to understand lipid-mediated processes and for the development of new strategies to diagnose, treat, and prevent human diseases caused by lipid-related processes. Genetically incorporated fluorescence protein-tagged lipid binding domains have been widely used as a probe or sensor for visualizing the spatiotemporal dynamics of various cellular lipids. Despite its experimental convenience and popularity, these methods do not provide quantitative information because fluorescence proteins do not undergo a spectral change upon lipid binding. To overcome this limitation, fluorescence resonance energy transfer (FRET)-based methods using a pair of fluorescence proteins, such as cyan and yellow proteins, have been devised. However, these methods generally suffer from low sensitivity and robustness in in situ lipid quantification. Furthermore, lipid sensors made of naturally occurring lipid binding domains may not be able to compete with those endogenous cellular proteins with higher affinity for and/or easier access to particular lipids. Although mass spectrometry-based lipid analysis offers higher sensitivity and provides the detailed structural information about lipids, including acyl chain compositions, the current method requires physical separation of lipids from cells and thus can provide neither spatial nor real-time temporal information.
Accordingly, lipid binding proteins that have a higher lipid selectivity and membrane affinity than wild-type lipid binding domains, minimal affinity for cellular proteins, and which can be easily tracked in a cell or vesicle are desired. More specifically, fluorescent lipid binding proteins (FLBPs) that are amenable to delivery into mammalian cells and subsequent imaging and quantitative studies are desired. Such compositions will allow for a better understanding of the molecular mechanisms underlying lipid turnover and lipid-related diseases.